Multiple communication terminals provided in a company or the like have typically formed a local area network (LAN) in recent years. In some cases, communication terminals forming a LAN are logically grouped regardless of how the terminals are physically connected to one another, so that multiple VLANs are formed in the LAN. If VLANs are formed, communication terminals belonging to an identical VLAN communicate with one another as if the communication terminals belonged to a LAN. Also, the communication terminals are distinguished from communication terminals belonging to a different VLAN regardless of how the former terminals and the latter terminals are physically connected to each other.
As for an configuration of a VLAN, for example, Japanese Laid-open Patent Publication No. 2005-20170 discloses a technology for efficiently routing a frame within a VLAN as well as between VLANs by providing an address conversion apparatus for converting a private IP address for independent use within a VLAN into a global IP address for common use outside the VLAN and vice versa. Specifically, in this related-art example, a single address conversion apparatus performs address conversion for each of multiple VLANs while changing an address conversion table for each VLAN.
Incidentally, communication terminals in a VLAN may be further grouped so as to form multiple subnets. Sometimes, these subnets are connected to one another via an L2 switch (Layer 2 switch). For example, as shown in FIG. 6, assume that a VLAN is divided into a subnet #1 and a subnet #2 and that an L2 switch 10 is provided for the subnet #1 and an L2 switch 20 is provided for the subnet #2. In this case, the subnets #1 and #2 are connected to each other by connecting the L2 switches 10 and 20.
If a communication terminal belonging to the subnet #1 transmits a frame to a communication terminal belonging to the subnet #2 in the VLAN configuration shown in FIG. 6, this frame is relayed by the L2 switches 10 and 20. That is, a frame to be transmitted between the subnets is always transmitted via a path between the L2 switches 10 and 20. Therefore, if different fee systems are applied to intra-subnet communications and inter-subnet communications, it is useful for the system to configure the VLAN having a plurality of subnets. Because whether the transmission of a frame is an intra-subnet transmission or an inter-subnet transmission is determined on the basis of whether the frame has been transmitted via the path between the L2 switches 10 and 20.
However, a problem here is that if an L2 switch is provided for each of multiple subnets, a port of each L2 switch is used to connect the L2 switches provided for the subnets to one another so that the number of available ports of each L2 switch is reduced.
Specifically, since the L2 switches 10 and 20 are connected to each other in the VLAN configuration shown in FIG. 6, one of the L2 switches must allocate one of ports thereof for connection to the other L2 switch. As a result, the number of ports of the L2 switch available for connection to communication terminals in the subnet is reduced by one. If the number of ports available for connection to communication terminals in one subnet must be increased, more L2 switches must be provided for the subnet. As a result, the cost required to form a VLAN is increased.
Also, an L2 switch is provided for each subnet in the VLAN configuration shown in FIG. 6; therefore, as the number of subnets is increased, the number of required L2 switches is also increased. Thus, the cost required to form a VLAN is further increased.
On the other hand, if a VLAN has a configuration in which multiple subnets are connected to one another via a single L2 switch, port-to-port connection between L2 switches becomes unnecessary and an increase in cost due to an increase in the number of L2 switches is prevented. However, if this configuration is adopted, whether the transmission of a frame is an intra-subnet transmission or an inter-subnet transmission is not determined on the basis of whether the frame has been transmitted via a particular path. That is, in the VLAN configuration shown in FIG. 6, the type of the transmission of a frame is determined on the basis of whether the frame has been transmitted via the path between the L2 switches 10 and 20; on the other hand, in the configuration in which a single L2 switch is provided between two subnets, the type of the transmission of a frame cannot be determined, since the frame is relayed by the L2 switch in both a case where the frame is transmitted within a subnet and a case where the frame is transmitted between subnets.